    1. Field of the Invention
The present invention relates to a method for reducing an oxygen component and a carbon component in fluoride.    2. Description of the Related Art
A lithographic process in the manufacture of a semiconductor is applied on an optical-purpose material such as a raw material of an optical system that constitutes a stepper device provided as an exposure device (e.g., a projective lens or an illumination lens) or a raw material, e.g., an optical fiber material or a coating material.
For instance, in the case of a lens material, an ArF excimer laser (193 nm) which is an exposure source of light having a short wavelength and an F2 excimer laser (157 nm) having an even shorter wavelength, have been used in connection with high integration semiconductor manufacturing. However, since a high-purity silica glass provided as a lens and typically used heretofore has a poor transmittance, it should not be used. Thus, fluorides such as calcium fluoride and barium fluoride, which have good transmittance properties even at short wavelength and strong laser durability, have been considered for use. Yet, a high purity monocrystal is absolutely essential for the manufacture of a fluoride lens made of calcium fluoride, barium fluoride, or the like. It has been known that various kinds of problems may be caused when an excess amount of oxygen or carbon is included as an impurity in the raw material.
The oxygen component, which may be incorporated into fluoride as moisture, can remain at the time of the manufacture, or an oxide is generated by the process of sintering, so that finally, at the time of lens formation. As such, (1) a reduction in transmittance, (2) a reduced laser durability, (3) a change coloring, and/or another change may be induced by the presence of oxygen. Thus, it has been said that the presence of oxygen can influence the transmittance of light. Therefore, a low concentration of oxygen in the fluoride component has been desired.
The carbon component can be caused from a crucible material at the time of sintering. Typically, an alumina crucible or a carbon crucible is used. When the alumina crucible is used, contamination with aluminum may occur. In general, therefore, the carbon crucible is used. However, carbon from a crucible material may remain in the fluoride. In the process of manufacturing the fluoride, furthermore, unreacted carbon (e.g., from carbonate as a raw material) generally used at the time of manufacturing the fluoride may remain in the product. This remaining carbon material in the fluoride product is said to be incorporated into a monocrystal at the time of manufacturing and tends to exert an adverse effect on coloring or the like.
Actually, at the time of manufacturing the monocrystal, the oxygen component is generally used as a scavenger where lead fluoride or zinc fluoride is used as a reactant. For instance, the lead fluoride performs a deoxidization process of the following reaction (A):CaO+PbF2-CaF2+PbO . . .   (A). 
The deoxidization process of the above (A) permits a decrease in the amount of oxygen. However, as the scavenger is added as a second component, the potential contamination generated therefrom makes the formation of a pure monocrystal essentially impossible. For minimizing the amount of the added scavenger as much as possible, there is a need of fluoride containing a smaller amount of the oxygen component in the raw material.